Transponders used in communication systems, such as satellite communication systems, tend to receive an incoming signal at a particular frequency and retransmit all or portions of the signal at a different frequency. Simple transponders can be thought of as repeaters since they typically receive an incoming signal and retransmit the entire received signal at a different frequency. More sophisticated transponders, however, may have the ability to break the received signal into multiple frequency bands or channels and then reconfigure the frequency bands or channels prior to retransmission.
Typically the more sophisticated digital transponders include five components: a digitizer module, a channelizer module, a switch module, a recombiner module and a digital to analog conversion module. The digitizer module digitizes the received analog signal, the channelizer divides the received signal into multiple channels, the switch module reconfigures the channels, the recombiner module combines the reconfigured channels to form one or more digital output signals, and the digital to analog conversion module converts the digital output signals into analog output signals.
Typically the channelizer module and the recombiner module are implemented as polyphase filter-based signal processors. These polyphase filter-based signal processors typically work with signals in complex format (in-phase and quadrature components separately) to minimize the dimensions of the polyphase filter banks. However, dealing with signals in complex format increases the number of conversion modules required and increases the complexity of the interfaces. This is particularly true with respect to the input and output ports of the digital processors.